JP-A-5-39763 discloses a pressure regulator for controlling pressure of fuel supplied to fuel injection valves. The pressure regulator has a spring chamber serving as a back pressure chamber. In this structure, set pressure of the pressure regulator is controlled by switching pressure in the spring chamber to either air intake pressure or the atmospheric pressure.
According to JP-A-7-293397, voltage applied to the fuel pump is controlled in accordance with the difference between target pressure and detection pressure, which indicates pressure of fuel supplied to the fuel injection valves.
In JP-A-5-39763, the controllable pressure range of the pressure regulator is narrow within the difference between the atmospheric pressure and the maximum negative pressure in the air intake pipe. Accordingly, the load applied to the fuel pump does not largely change. Consequently, change in the electricity consumption of the fuel pump is small, and the fuel pump may consume a large amount of electricity in an operating condition where the engine does not require high pressure fuel.
According to JP-A-7-293397, voltage applied to the fuel pump is controlled in accordance with the difference between the detection pressure and the target pressure, so that power consumption of the fuel pump changes in accordance with the operating condition of the engine. Thus, power consumption of the fuel pump can be reduced.
However, in general, an electric fuel pump is designed to produce optimum efficiency when the fuel pump is applied with the maximum voltage. Therefore, when the voltage applied to the fuel pump decreases, the efficiency of the fuel pump decreases. The efficiency η of the fuel pump is defined by: η=(P×Q)/(I×V). Here, driving current supplied to an electric motor of the fuel pump is I. Voltage applied to the electric motor of the fuel pump is V. Discharge pressure of the fuel pump is P. A discharge amount of the fuel pump is Q. In JP-A-7-293397, the voltage applied to the fuel pump is controlled in accordance with the difference between the detection pressure and the target pressure, so that the power consumption of the fuel pump can be decreased. However, the efficiency of the fuel pump decreases.
It is required to further enhance atomization of fuel, which is injected from fuel injection valves, in order to reduce unburned component in exhaust gas emitted from an engine or in order to facilitate engine start in a low temperature condition or a high temperature condition. In order to enhance atomization of fuel, it is conceived effective that, for example, increasing pressure of fuel supplied to the engine, not only improving fuel injection valves such as a shape of an injection nozzle thereof. In JP-A-5-39763 and JP-A-7-293397, pressure of fuel supplied to fuel injection valves can be increased by jumbo-sized the fuel pump to enhance discharge pressure of the fuel pump. However, when the fuel pump is jumbo-sized, electricity consumption becomes large, and efficiency of the fuel pump decreases.
As disclosed in JP-A-2003-293883, when two fuel pumps are connected in series, pressure of fuel supplied to the engine can be enhanced without jumbo-sized the fuel pump. Thus, the fuel pumps need not be jumbo-sized by driving both the two fuel pumps connected in series. However, even in this structure, electricity consumption becomes large, and efficiency of the fuel pump decreases in each of the fuel pumps.